In case of mechanical respiration imitating natural respiration, the inspiration and the expiration take place via a single-channel tube introduced into the patient's trachea. Splitting into separate flow paths is performed in the vicinity of the patient via a Y-piece, and these separate flow paths can be released via actuated valves and thus make possible a desired uncoupling of the volume flows generated during expiration and inspiration, corresponding to the mode of respiration selected.
The uncoupling of the volume flows can never be complete for anatomic and technological reasons. For anatomic reasons, there is a dead volume, which is determined, among other things, by the volume of the trachea, even during natural breathing. In case of artificial respiration, the volume of the tube is a technological dead volume. If additional volume areas through which both volume flows flow are present between the outer end of the tube and the beginning of the separate flow guiding, these automatically increase the dead volume during respiration. To attain a low flow resistance during respiration, the cross section is usually expanded already in the transition area between the tube and the Y-piece. As a result, the Y-piece itself may already contribute appreciably to the technological dead volume. If additional flow-carrying components, for example, sensor heads with flow sensors, are inserted between the tube and the Y-piece, these also increase the dead volume of the respiration unit.
The dead volume may, as a rule, be tolerated if the tidal volume is markedly larger than the dead volume. However, cases in which a small tidal volume must suffice for complete respiration frequently occur during mechanical respiration in medicine. This may be due to various impairments in the functionality of the respiratory system or a small lung volume. The latter occurs mostly in neonatology. It is desirable in such cases to keep the dead volume as small as possible during artificial respiration.
It is frequently desirable to obtain the data necessary for a “real-time monitoring” of a patient possibly in the vicinity of the patient from a measurement of the gas flows being sent through the respiration unit. If flow sensors or sensors for measuring certain properties of flowing gases are directly exposed to the gas flow to be measured, they frequently yield evaluable signals only if uniform flow conditions prevail in their immediate environment and the sensors themselves do not affect the gas flow in an undefined manner. Sensors are therefore sometimes accommodated in special components which are optimized only with the aim of guaranteeing defined flow conditions in the environment of the sensor. However, the integration of additional components implies an increased effort for installation, it possibly increases the dead volume and may lead to problems in terms of optimization in the overall system if individual components are optimized concerning their individual functions but not for an optimal cooperation. One example of such an optimization concerning individual functions is the design of conventional Y-pieces. These contain an unbranched area, which can be called a base area, which is flown through in both directions and whose dimensioning is determined essentially by the conditions of connection to the tube or to a tube adapter. Furthermore, Y-pieces comprise two outflow pipes for the connection of separate flexible tubes, which are flown through in one direction only. Flexibility problems are usually the most important aspects in designing these outflow pipes. The connection sites must have a certain minimum distance for the possibility of handling and shall permit rapid mounting. As a result, difficult-to-define flow conditions may arise in the base area of the Y-pieces, where the separate flow paths are merged.
A device is known from WO 84/01704 as an example of the integration of additional components for measurement purposes, in which venturi tubes are arranged behind the Y-piece in the flow paths already extending separately, and the pressures occurring at the reduction of the area of these venturi tubes are monitored for differential pressure measurement. However, the measured values are not obtained particularly close to the patient because of the arrangement behind the Y-piece.